The need to place a liquid sample into a centrifuge rotor and subsequently remove it after centrifugation has lead to the development of many types of tubes and associated means of sealing the tubes. The field of ultracentrifugation where rotational speeds may range from 20,000 to over 100,000 revolutions per minute with corresponding g-fields from 50,000 to over 650,000 has caused the development of principally two types of tube sealing methods: mechanical and hermetical or heat seal. Sealing methods have been refined to meet the demands of increasing speeds, g-fields and the corresponding hydrostatic pressures generated inside the tubes. Often times the development of a particular sealing mechanism has resulted in an adequate seal but has encumbered the user of the centrifuge rotor and tube with an awkward method consisting of complicated and costly hardware to make the seal. A discussion of the many forms of seals is presented in S. T. Nielsen, "Centrifuge Tube Having Removable Crown and Swage Fitting", U.S. Pat. No. 4,537,320.
In U.S. Pat. No. 4,537,320, the sealing process is simplified and requires a minimum of special tools. A centrifuge tube seal is achieved by an outward swaging of a protruding stem by means of a conical seal plug seat against a corresponding conical surface of a support crown. Various B means are described for holding the flexible protruding stem in position while the swaged seal is accomplished. A swaged compression seal is achieved by the application of torque to the swage plug by a simple wrench. One limitation that has been realized with this method and article is the practical limitation on size of the components as the invention is applied to the many smaller sizes of tubes and centrifuge rotors. Because the number of existing rotors in the field is vast, it is desirable that a particular centrifuge tube and seal mechanism be designed for use in existing centrifuge rotors. Existing rotors generally have a limited head space between the top of the tube hole and the lid of rotor as this space continues to be designed for conventional tube caps as described in L. C. Marks, "Centrifuge Test Tube Cap", U.S. Pat. No. 3,459,369. It is generally desirable to maximize the internal volume of a centrifuge tube by minimizing the volume of a given seal assembly. The seal plug and crown plug assembly described in U.S. Pat. No. 4,537,320 has found practical application in such a way as to make the seal plug a common size with varying support crowns for all the rotors in which it is used. Unfortunately, this approach has lead to tubes with approximately 10% less internal volume than the conventional capping or sealing methods. The tube stem has been kept a common size so that the user can standardize the types of syringes and loading procedures used. The result has been that there is a practical minimum size where the standard seal plug can be used without the design of smaller tube stems and seal plugs. The swage seal has been applied to tube diameters of 5/8 inch nominal and larger. For tube sizes significantly smaller than 5/8 inch in diameter, for example 1/2 inch in diameter, swage seal designs would require smaller, nonstandard tube stems, seal plugs and corresponding support crowns so that the seal assembly could be contained within an existing centrifuge rotor with a fixed head space. Because the swage seal is accomplished through an outward swaging of the tube stem, the diameter of the seal surface is larger than the original unswaged stem, B and the support crown and seal plug are correspondingly larger than the stem diameter. Application of this seal approach to smaller diameter tubes would require small stem diameters and would make filling of the tube quite difficult. If these components were developed for smaller diameter tubes, there would come a point where the features of the swage seal assembly would become very small and in some cases delicate or difficult to fabricate. Further, in practice it is has been found that there is a limit to the number of times a tube can be reliably unsealed and resealed since abrasion of the swaged seal surface of the tube occurs at each reuse.
The hermetical or heat sealed tube described in S. T. Nielsen, "Integral One-Piece Centrifuge Tube", U.S. Pat. No. 4,301,963 requires the application of heat, the use of relatively complex sealing instruments and hardware that can only be used once since the integral stem which is melted during the sealing process is cut off after centrifugation to gain access to the contents of the tube. Furthermore, problems in heat sealing may occur and any attempts to reseal the unused tube are often unsuccessful because additional, integral stem material is not available to reestablish the seal. If a sealing error is made, the contents of the tube, which may consist of many layers of density gradients and biological samples must be tediously removed and transferred to another tube for a second attempt at sealing with attendant loss of time, possible contamination or loss of sample and added cost.
A method for a mechanical tube seal has been developed by the E. I. du Pont de Nemours Co., Inc. of Wilmington, DE, and displayed under the trademark of "Crimp Seal". Like the hermetically sealed tube, it is designed to be a one-time use seal. A flexible plastic tube stem is plugged with a rubber bushing which extends into the tube stem and a ductile aluminum cap is placed over the top of the rubber bushing and extends down over the sides of the tube stem. A crimping device permanently deforms the aluminum cap against the tube stem causing the rubber bushing to be compressed and permanently constrained on its sides and top. The compression and interference fit of the rubber bushing with respect to the inside of the tube stem is said to be great enough to cause a liquid seal during centrifugation. The crimping process is very similar to that of a septum sealer which is used to crimp rubber septums to the top of glass vials for the storage of chemical and biological samples. The Crimp Seal in ultracentrifugation is considered to be a one-time seal. It would be most difficult to remove the ductile aluminum cap and rubber bushing which may be significantly and permanently deformed. If an insufficient crimp is applied because of improper adjustments of the crimping collet, it is difficult to cut away the aluminum cap without shaking the tube which could disturb the carefully layered density gradients and biological samples. Generally, as with the hermetically sealed tube, errors in the sealing process require the cutting way of the stem or puncturing of the tube, and the subsequent transfer of the tube contents to another tube for another attempt at sealing. In any case, even if a successful seal is made and centrifugation completed, retrieval of the tube contents is made by cutting the stem away.